Perhaps the most widely used directional gain antenna system is a yagi antenna system. The most common yagi antenna system comprises a radiator, a reflector, and a director. These three components are typically arranged such that the director element is in the front, the radiator is behind the director, and the reflector is behind the radiator. In general, the director element is the shortest element while the reflector is the longest. The length of the elements and the distances between them determine the radiating power of the antenna system.
The radiator of a yagi antenna system is driven by the antenna feed line. The director and reflector in a yagi antenna system are considered parasitic elements. A parasitic element obtains its power through coupling with a driven element, as opposed to receiving it by connection to the power source. In a typical yagi antenna system, the director and reflector obtain their power by coupling with the radiator. In general, yagi antenna systems utilize a gamma match, T-match, or stub-type matching network at the driven element to obtain an electrical/mechanical impedance matching network for the antenna system.
A log periodic antenna system is a system of driven elements, designed to be operated over a wide range of frequencies. The advantage of a log periodic antenna system is that it exhibits essentially constant characteristics over the frequency range (i.e. the same radiation resistance, front-to-back ratio and approximately the same gain). The most common log periodic antenna system is the log periodic dipole array (LPDA).
A LPDA consists of several dipole elements which are each of selected different lengths and different relative spacings. Typically, a distributive type feeder system is used to excite the individual dipole elements. Each dipole element is driven with a phase shift of 180.degree. by switching or alternating element connections. In general, a balun is used at the antenna system feed point to impedance match the antenna system to the antenna transmission line.
One technique for increasing both the gain and the front-to-back ratio for a specific frequency within a passband is to add parasitic elements to a LPDA to form a logarithmic yagi antenna system. As shown in prior art FIGS. 1 and 2, the LPDA-Yagi combination utilizes a LPDA group of driven elements, together with parasitic elements at normal yagi spacings from the end elements of the LPDA.
The method of feeding a hybrid logarithmic yagi antenna system is generally identical to that of feeding a LPDA antenna system without the parasitic elements. Typically a balanced feeder is required for each log-cell element, and all adjacent elements are fed with a 180.degree. phase shift at the connection points for the alternating dipole element feed lines. A balun is connected at the log-cell input terminals to provide the balanced feed and impedance matching.
Generally, a boom and mast are used to mount a logarithmic yagi antenna system to a structure, such as a building. The LPDA, reflector, and director are secured to the boom and the mast is mechanically connected at a central portion of the boom for mounting the entire antenna system to a structure.
The requirement of a balun for providing the balanced feed and impedance matching and a boom and mast to mount the antenna system unnecessarily increases the complexity, size and cost of the antenna system. Furthermore, installation of such an antenna system is complicated since the balun must be tuned for each individual antenna system in order for the antenna system to work at peak efficiency.
Accordingly, a need arises for a hybrid logarithmic yagi antenna system which is simple, compact and cost effective. The antenna system should be pre-tuned and easy to install, while providing high gain, high front-to-back ratio, and wide band operation.